A lithium ion secondary battery having high energy density has recently attracted much attention as a stationary power source for small electronic devices, hybrid cars, electric vehicles, and power storages. Of such batteries, much anticipation surrounds the lithium ion secondary battery with an inorganic solid electrolyte as a safe battery since it is free from risks such as leakage of an organic electrolytic solution and gas generation. Further, a lithium battery using a solid electrolyte is expected to attain a longer life since it is less subject to side reactions other than a battery reaction as compared to batteries using electrolytic solution. Furthermore, since it is easy to form an all-solid battery using an inorganic solid electrolyte by layering an electrode and an electrolyte layer, the all-solid battery enables to reduce the production cost and also to form a bipolar type battery. Accordingly, a higher energy density is expected as compared to batteries using a liquid-based electrolyte. However, since a material having a high oxidizing power is used for a positive electrode and a material having a high reducing property is used for a negative electrode in a lithium ion battery having high electromotive force, it is necessary for the materials and the solid electrolyte to be stable when they are brought into contact with each other.
Perovskite compounds which have a high lithium ionic conductivity and are easily obtainable by a solid phase reaction in an air atmosphere are attracting attention. A representative example of such a material is La0.67−XLi3XTiO3. One of the compounds having a composition of X=0.11 shows a high lithium ionic conductivity at room temperature Of 1.5×10−3 S/cm, which is excellent for an oxide-based solid electrolyte. On the other hand, since titanium, which is one of the constituent elements, is easily reduced, there has been a problem that the electrolyte reacts with the negative electrode material having a strong reducing property to exhibit electronic conductivity due to the reduction of tetravalent titanium into trivalent titanium. Since the conductivity of the electrolyte other than the ionic conductivity can entail internal short-circuiting of the battery, the problem is a big issue in terms of its practical application. Therefore, a method of replacing a part of the constituent elements with another element, which has higher reduction-resistant than titanium, has been tried. However, the replacement of the constituent element causes a considerable degradation in the lithium ionic conductivity to about 1/10 to 1/100.